1. Field of the Invention
This invention relates to a method of manufacturing a hollow fiber, and more particularly to a method of manufacturing a hollow fiber having favorable selective permeability.
2. Description of the Prior Art
Recently, hollow fibers are widely used in various fields. A method of manufacturing a hollow fiber has been investigated in order to impart various performances to the hollow fibers.
Examples of the successful use of the hollow fibers are a hollow-fiber type artificial kidney, a hollow-fiber type module for reverse osmosis, a hollow-fiber type ultrafilteration module and a hollow-fiber type reverse-osmosis condenser.
The artificial kidney has been used for a patient suffering from renal failure. Metabolic waste such as urea and uric acid are selectively removed from the blood of the patient by the artificial kidney. A well-balanced ultrafiltrating property or dewatering property with the removable performances of other metabolic waste is required for the artificial kidney. The hollow-fiber is also used for reverse osmosis, for example, for desalting sea water. A good water-flux and rejecting property against salt are required for reverse osmosis. The hollow-fiber type module is also used for treating polluted water from a factory, or for producing sterile water. The hollow-fiber type reverse-osmosis condenser is also used for food industry, for condensing juice. This method is advantageous in view of minimum loss of vitamins during the condensation process and, because heat is not used in this method, it saves energy.
In using hollow fibers, a fluid to be treated flows through the hollow portions of the hollow fibers. Accordingly, a hollow portion should extend longitudinally through the whole length of each fiber. The membrane of the hollow fiber should not be even partially broken. Further, it is required that the thickness of the membrane of the hollow fiber be:(a) uniform over the fiber's whole length fiber and (b) as small as possible with sufficient mechanical strength.
A conventional method of manufacturing the hollow fiber has been that a spinning solution (dope) is extruded from an annular slit of a double pipe orifice, to form a sheath solution, and simultaneously, a gaseous or liquid fluid is extruded from the inner pipe of the double pipe orifice to form a core fluid. So far, the use of liquid as core fluid is superior to that of gaseous fluid for stable spinning. The core liquid known so far can be classified into two categories.
The first one is a liquid having no compatibility with a spinning solution (sheath solution). For example, a cuprammonium solution of a cellulose is extruded as a sheath solution from the double pipe orifice, and simultaneously an organic solvent having no compatibility with water is extruded as a core solution from the inner pipe orifice (Japanese Patent Opening Nos. 40168/1975, 59518/1975 and 31912/1974). In these patents, the uses of octyl alcohol, benzene, toluene, styrene, light oil, kelosene, perchloroethylene, trichloroethylene, methylchloroform, and liquid paraffin are disclosed as core solution. The spun-dope filament is not miscible with the core solution, but the dope filament and core solution form separate phases. Thus, the mutual diffusion between the solvent of the sheath solution and the core solution is suppressed. Accordingly, gellation or coagulation of the spinning dope filament does not develope. For this reason, the spinnability is satisfactory and a high speed of spinning is possible. However, the above-described method has a disadvantage that it requires much labor and time to remove the core solution from the manufactured hollow fiber, and the removal of the core liquid from the hollow fiber is always imperfect, because the core solution is essentially incompatible with water. These inventors previously proposed a novel method of manufacturing hollow fiber in which an aqueous salt solution containing a sufficient amount of salt is used as core liquid, utilizing a phase separation between the core and sheath solutions due to salting-out effect (Japanese Patent Applications No. 77447/1976 and No. 143115/1976). This method is included in the first category in a sense of using a core solution which is incompatible with the sheath solution.
The above-described core liquid is often called "non-coagulating liquid". However, an essential point is that the core liquid is not compatible with a sheath liquid. In other words, a core liquid is separated from a sheath liquid, therefore, the mutual diffusion is suppressed. In a dry-jet-wet spinning process using the above-described method, the spun-dope filament runs through a gaseous space of an appropriate length before it is introduced into a coagulating both. The gellation of the spun filament is not developed by the core liquid in the passage that the filament is running through the gaseous space. Accordingly, the spinnability is excellent. Thus, the high speed spinning is possible without any trouble. Another method is proposed (Japanese Patent opening No. 132218/1975) in which the core solvent is essentially the same as the sheath solvent, but contains a water soluble polymer. The idea is that a polymer solution develops phase separation when different kinds of polymers are contained in the solution. Since the polymers in the core liquid and the sheath liquid are different, these solutions becomes imcompatible each other to form two separated phases. However, according to this method, it is very difficult to remove the core polymer from the manufactured hollow fiber. Accordingly, this method is practically impossible.
The second category is that water or an aqueous solution is used as the core liquid. The aqueous solution is usually a mixture of water and the solvent for the sheath solution. Diffusion rapidly occurs between the sheath solution and the core liquid. Since coagulating function of water is highly effective, the spun-dope filament is almost instantaneously gelled by the action of the core liquid immediately after being spun. Accordingly, the spinnability is very poor, and the draft ratio, which is the ratio of linear speed of spun-dope filament to take-up speed on a take-up roll, is also very small. Thus, the spinning speed is at most 5 to 15 meters per minute. However, the resultant hollow fiber has characteristic properties, for example, in selective permeability. The hollow fiber obtained by this method is known to be suitable for ultrafiltration. Hollow fibers from cellulose acetate, and vinyl halide (Japanese Patent Opening No. 80686/1976) and from acrylonitrile polymer (Japanese Patent Opening No. 6532/1974) are proposed using the above method.
When the above-described core liquid is used, the spinning speed is low as already described, because the spinning solution is rapidly gelled due to the rapid coagulating action of the core liquid. However, structure having high water-flux is imparted to the resultant hollow fiber, which is useful as a ultrafiltration membrane. A method to improve the spinning speed is proposed in the Japanese Patent Opening No. 13424/1974 in which the gellation of the spun solution of cuprammonium cellulose is suppressed by using chilled water as a core solution. It should be noted that such core solutions act to coagulate and gellate the spun filament.